The present invention relates to magnesium aluminate spinel refractory shapes and particularly bricks particularly suited for use in linings of rotary cement kilns which are low cost without sacrificing the necessary properties of low thermal conductivity, low thermal expansion, resistance to corrosion, high spall resistance, and high hot strength.
For a number of years magnesite-chrome refractory shapes, such as brick, provided excellent service and were extensively used as linings in metal working vessels and kilns such as rotary cement kilns. Because of the formation of hexavalent chrome which is classified as a hazardous substance by the federal government and which consequently posed a number of problems; including disposal of used magnesite-chrome brick, the use of such brick has fallen into disfavor.
Efforts to utilize other types of brick, particularly for the harsh environment of the rotary cement kiln operation, have not been entirely successful. However, one type of brick which is being more widely used is magnesite-spinel brick. However, the drawback of magnesite-spinel brick is that they are more expensive than the magnesite-chrome brick traditionally used. Even with the knowledge of the potential dangers of hexavalent chrome formation in magnesite-chrome systems, the magnesite-chrome brick still dominates all other types of brick used in the rotary cement kiln industry. The higher cost of magnesite-spinel brick stemmed mainly from the raw materials used to make the brick as well as some processing costs. Alumina is more expensive than chrome ore and is being used to make in situ spinel brick or the alumina is blended with magnesia then pre-sintered or fused to form spinel grains which were then utilized in making the brick. Such pre-sintering or fusing of the grain is an extra and expensive processing step which again raises the cost of the magnesite-spinel brick.
Efforts to substantially lower the cost of magnesite-spinel brick while maintaining performance level have not been heretofore successful.